1. Field of the Invention
The present invention relates to an image forming apparatus that forms a color image by superimposing a plurality of toner images of different colors.
2. Description of the Related Art
In general, a tandem-type color-image forming apparatus that includes a plurality of photosensitive elements for forming a color image has disadvantages such as a large apparatus size due to a large-sized image forming engine, a complex configuration, and a high cost. To overcome the disadvantages, a rotary-type color-image forming apparatus that includes a single photosensitive element is known, which is disclosed, for example, in Japanese Patent Application Laid-open No. H08-087179 and Japanese Patent Application Laid-open No. H10-003191. In the rotary-type color-image forming apparatus, toner images of a plurality of colors are sequentially formed one after another on the photosensitive element, and then collectively transferred onto a recording medium such as a recording paper.
The color-image forming apparatus reproduces a color image according to subtractive color mixture of cyan, yellow, magenta, and black by appropriately superimposing toner images of cyan, yellow, magenta, and black.
A method of forming a color image in an image forming apparatus disclosed in Japanese Patent Publication No. 3014168 by superimposing a plurality of toner images of different colors on a photosensitive element is as follows. A charging unit uniformly charges the surface of the photosensitive element and an exposure unit exposes an image area for the first color toner image on the photosensitive element to reduce potential of the image area thereby forming an electrostatic latent image for the first color toner image. The latent image is developed with a toner of the first color having the same polarity as the surface of the photosensitive element, so that a toner image of the first color is formed. Thereafter, charging and exposing are performed in the same manner on the photosensitive element on which the first color toner image is formed, thereby forming a latent image for a second color toner image. The latent image is developed with a toner of the second color in the same manner as the above procedure to form the second color toner image. This process is repeated for the number of colors of toner.
When developing a latent image for the second or later color toner image, an amount of toner that adheres to the surface of the photosensitive element is significantly different between an area on which a toner image has already been formed (hereinafter, toner-image formed area) and an area on which a toner image has not already been formed (hereinafter, non-toner-image formed area), which causes uneven density in a toner image to be developed.
One reason for this is that when forming a latent image by exposing the surface of the photosensitive element, potential (post-exposure potential) is not reduced in the toner-image formed area due to a charge amount of the toner image in the same manner as the non-toner-image formed area. This causes a development potential, which is a difference between the post-exposure potential and a development bias, to differ between the toner-image formed area and the non-toner-image formed area. Specifically, the development potential of the toner-image formed area is lower than that of the non-toner-image formed area by the amount of charges of the toner image. Therefore, the adhering amount of toner is different between the toner-image formed area and the non-toner-image formed area.
Japanese Patent Application Laid-open No. H08-286456 discloses an image forming apparatus in which a photosensitive element and a first color toner image on the photosensitive element are neutralized by a neutralizing unit during a period from when the toner image is developed on the photosensitive element until when the photosensitive element is charged again by a charging unit for forming a second color toner image of the next color. The neutralizing unit neutralizes the photosensitive element and the first color toner image by supplying charges having a polarity opposite to that for charging the photosensitive element and a toner image by the charging unit.
However, when the photosensitive element is charged by the charging unit for the second color toner image, the neutralized first color toner image is also charged to the same polarity again. Therefore, it is difficult to reduce the influence of the charges of the first color toner image on forming the second color toner image.